A superconducting wire based on metal such as NbTi used in the related art is provided in the form of round wire or rectangular wire, and the degree of freedom in shape is high. On the contrary, in an oxide superconductor which is based on bismuth (Bi) or yttrium (Y), or the like and of which the critical temperature is in a range of about 90 K to 100 K, an oxide superconducting layer is formed of ceramic, and the structure thereof has a tape shape of which the aspect ratio is great.
The oxide superconducting layer is a kind of ceramic, and the rare earth oxide-based superconducting wire which has been known to be based on Y has a structure in which a plurality of thin films are stacked on a metal substrate provided as a tension member. For example, the rare earth oxide-based oxide superconducting wire employs a structure in which an oxide superconducting layer is stacked on heat resistant tape-shaped metal substrate such as hastelloy (manufactured by Haynes International, Inc., product name) or the like via an intermediate layer in which the crystal orientation is controlled, and a stabilized layer formed of the metal material such as Cu is stacked on the oxide superconducting layer.
With respect to the superconducting coil obtained by using the rare earth oxide-based superconducting wire, recently, it has been indicated that peeling stress acts in the thickness direction of the superconducting wire when being cooled due to the linear expansion coefficient difference or the shrinkage rate difference (since linear expansion coefficient becomes non-linear in low temperature, ratio of lengths in room temperature and low temperature is indicated by percentage) between the impregnated resin and copper or hastelloy (manufactured by Haynes International, Inc., product name) that configures a superconducting wire, when the coil is formed, such that the superconducting characteristic may be deteriorated after the formation of the coil.
Recently, as a measure for decreasing the peeling stress described above, coil structures disclosed in PTL 1 or 2 described below have been suggested.
The structure disclosed PTL 1 is a structure in which an insulating layer is coated on the whole circumferential surface of the superconducting wire and a mold releasing material layer is formed only on a portion of the outer surface of the insulating layer as a measure for decreasing peeling stress to the rare earth oxide-based superconducting wire, and particularly, a technique in which the mold releasing material layer is applied only to a portion in which the stress is strong in the radial direction of the superconducting coil is also disclosed.
In PTL 1, a composite superconducting wire in which the whole circumference of an oxide superconducting wire formed of a flat material is coated with an insulating material layer so as to be provided along a mold releasing material layer on one surface side thereof is disclosed. In addition, a superconducting coil having a structure in which the composite superconducting wire is subjected to coil processing, and is hardened by a curable resin layer that is cured by impregnating a thermosetting synthetic resin such as an epoxy resin.
Subsequently, in the technique disclosed in PTL 2, a structure in which a superconducting coil portion obtained by winding a thin film superconducting wire having a multilayered structure on the external circumference of an annular winding core in a concentric circular shape is included and mold releasing portions are provided on respective boundary portions of an inner portion, an intermediate portion, and an outer portion of the superconducting coil portion so that the stress can be released is suggested.
According to the structure disclosed in PTL 2, a structure in which the bonding strength of the superconducting coil portion in the radial direction is weaker than that of the other portions is partially introduced, the stress can be released in a portion in which the bonding strength is weak, and thus the decrease of the peeling stress on the superconducting wire itself is attempted.